TL1A exists in both a membrane bound- and a soluble form and is secreted from antigen presenting cells (APCs) such as monocytes, dendritic cells and macrophages in response to stimulation with immune complexes, bacteria or cytokines (TNF-alfa and IL-1beta). Membrane bound TL1A has also been described in T cells. Recently, a new isoform of soluble TL1A (TL1A(V84-L251)) was discovered with functional differences to TL1A(L72-L251. It's unknown whether this isoform is present in psoriatic skin.